The present disclosure relates to a laser scanning device for scanning a scanned surface with a light beam emitted from a light source, and to an image forming apparatus including a laser scanning device.
Electrophotographic image forming apparatuses include a laser scanning device configured to emit a light beam for scanning a photoconductor. The laser scanning device includes a deflector that deflects the light beam so that the deflected light beam scans a scanned surface of the photoconductor. As the deflector, there is known one using a polygon mirror having a plurality of reflection surfaces, or one using an oscillation mirror (also called a resonance mirror) such as a MEMS mirror in which a reflection surface makes a sinusoidal oscillation in a reciprocating manner to deflect the light beam. In recent years, an oscillation mirror having a small error and a small driving load has been used to realize a high-speed scanning.
In the oscillation mirror, since the reciprocating operation of the reflection surface is sinusoidally driven, the operation speed changes in synchronization with the sinusoidal waves in the oscillation range. As a result, laser scanning devices using the oscillation mirror include a curved lens having an arc sine property (hereinafter, such a curved lens is referred to as an “arc sine lens”) so that the light beam moves on the scanned surface at a constant speed in the scanning direction.
The arc sine lens enables a light beam to scan the scanned surface at an equal speed, but the spot diameter (also referred to as a beam diameter) of the light beam increases as it moves away from an optical axis of the lens. In other words, as the field angle of the oscillation mirror with respect to the optical axis increases, the spot diameter of the light beam on the scanned surface increases. As a conventional technique coping with the problem, there is known a correction technique for aligning the size of the spot diameters by adjusting the amount of light at each scanning position on the scanned surface. In addition, as another conventional technique, there is known a correction technique for aligning the size of the spot diameters by decreasing the exposure time period and increasing the light intensity as the field angle of the oscillation mirror increases. It is noted that in the present specification, the spot diameter refers to a diameter of a light flux at a point where the light intensity is 1/e2 (=13.5%) of the peak value of the light intensity of the light beam irradiated on the scanned surface.